10 Answers
10

As Matthew Read mentions, the hardware, your location, and the other fiddly bits about actual communication are important to the discussion. But, you also need to consider what you're doing with the phone.

Are you retrieving largish amounts of data often? If so, Wifi may be less drain on the battery by letting you finish sooner - remember the screen drains power while you're waiting for 3G to finish what WiFi may have finished much sooner. (But that depends on what brightness level you have your phone set to.)

In short, set your phone up the way you like to use it, in the place you like to use it. Then run the best test you can to see which uses more battery. Keep everything as much the same during the tests as possible. (Know exactly which apps are running, plan what data to use, and time how long you use the data after it's retrieved.)

I appreciate the points. I think I was curious in general, maybe per unit of data transmitted? But as others have noted, the screen backlighting on longer while you wait on 3G vs WiFi is a real issue as well.
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geoffcFeb 1 '11 at 23:27

I never really did a precise comparative test but it is clear that wifi uses way less power than the 3G data connection. So I would recommend to use wifi whenever possible if the goal is to extend battery life.

The only exception I would see is if you often use the phone but for a very short moment at a time, the delay it takes for the wifi to connect can be a hassle. In this case only I would deactivate the wifi.

@Matthew Read: That is interesting! Can you quantify it any better than that? (I am not sure how, other than perhaps some silly web page with a constant refresh set, and leaving the phone doing nothing but the web page over Wifi vs 3G perhaps.
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geoffcFeb 1 '11 at 23:24

Well I can't test right now (I no longer have a data plan), but when I got my phone I did experiment a bit. I consistently got about 30% battery drain overnight with my phone on 3G, and 10% on WiFi. I did probably a week of each, so while it's kinda unscientific I think the results are valid -- at least for my hardware, software, location, etc.
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Matthew Read♦Feb 2 '11 at 1:58

@Matthew Read: Isn't 30% drain on 3G higher than 10% on wifi? Maybe you meant the opposite. And by default, even when using wifi, a couple of minutes after the phone goes to sleep, the wifi connection also goes to sleep and it returns to 3G. If you select the option to keep wifi on even when the phone is going on sleep, then I would believe it takes more battery because the wifi chip stays on all the time for nothing.
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jmbouffardFeb 2 '11 at 12:21

I can't remember where I read it but I read that 3G will use more power when transfering data but wifi will use more when idling. I tend to keep wifi on, but if I'm running low on power I'll turn it off to avoid the power requirements of scanning for new networks. Also as others have posted, 2G will use less power than both unless you're downloading large files!

i also read this somewhere. but cant remember where.My experience also confirms this.i.e if you just need to be online and recieve minor chat msgs etc(most of the time idling,then switch to 3g or better 2g). If you need to transfer huge data and are near a wifi hotspot, use that instead.This will maximize the battery.
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ashishsonyJun 17 '13 at 9:43

It depends.

Try with your particular phone. Maybe download a large file and see how much the battery drains, then repeat for the other data type. Factors include how many cell towers are nearby, how many packets your WiFi drops, your specific chipset and radio hardware, software drivers, and the like.

I had hoped it was a little more general than that. Alas.
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geoffcFeb 1 '11 at 23:25

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Yes, this is my understanding as well. There are so many scenerios to test to get a handle on your device but I'd specifically test the following: 1. Active use: downloading. 2. Active use: uploading. 3. Passive use (receiving background syncs) in good signal area (where you have uninterupted wifi and 3g). 4. Passive use with no reception so yor device is constantly looking for a connection (you'll have to turn off 3g and wifi respectively). This should give you a good idea how your wifi and 3g consume power.
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MattFeb 2 '11 at 9:54

In my experience (HTC Hero and now HTC Desire HD), wifi uses way less power than 3G. My first port of call for power saving is to turn off 3G on my handset if I either don't need mobile data or there is wifi available.

While it was true in previous generations of phones that Wifi used more power in idle, it's less true in later generations(think HTC Desire Z and up), with latests wifi chips.

If you are using data, Wifi is then preferable: uses less power, and can have more bandwidth and less latencies depending on the provider… Although public hotspots tend to match latencies and bandwidth of 3G networks… (on purpose?)

The main battery drain happens when there is actual data transfer. Since the speed of WiFi is much faster compared to 3G, it uses less power to download the same amount of data since it finishes downloading faster.

Also the transmuting power of WiFi is usually smaller than this of a 3G radio since the access point is much closer and the signal much stronger than the usual 3G tower. They tend to be less dense than 2G towers and the phone will need to transmit with more power in order to keep reasonable signal strength with 3G.

Your first paragraph is only true if WiFi does not use a fair amount more power per second than 3G, and most of the time it does.
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Matthew Read♦Feb 1 '11 at 17:33

You posted that it depends. And so does in this case. In the long run (for instance in the span of several days) if you use only 3G, you will need to recharge the phone more than if you used WiFi for most of the data needs. At least that is my observation in a quite covered 3G area (with a few black spots). Also if I used 2G it lasted more than either WiFi or 3G but this is irrelevant to the question.
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StoinovFeb 1 '11 at 20:50

Hence I used "most of the time" not "always".
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Matthew Read♦Feb 2 '11 at 19:20

I can confirm the experiences of 3G using more power than wifi, and I do believe it is, in fact, stated by Google somewhere in the Android FAQ.

The technical reason probably has to do with the fact that there is much more negotiation going on for 3G, where there could be many radio towers involved, and where they will actually instruct the phone to beef up transmission power if needed. Also, it goes without saying, that there ought to be a difference considering wifi has a coverage area limited to some 20-100 meters depending on equipment and obstacles; whereas 3G operates over a much longer distance up around several Km.

Both operate within a fairly similar frequency band of 1.9GHz to 2.4GHz. In general, the higher the frequency, the more data can be transferred but it also takes more power to get a signal through obstacles. This is also why it's sometimes recommended to turn off 3G (rely on only 2G) in order to conserve battery, since 2G networks usually operate at a much lower frequency band around 800-1000MHz.

Misses the point of the question, but I understand your point.
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geoffcFeb 1 '11 at 23:28

@geoffc Not sure how it misses the point. Sometimes you have to think outside of the box (of 3G vs Wifi) and look at other (battery saving) options. 2G is an adequate speed if you are looking to save battery life in the long run.
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Bryan Denny♦Feb 2 '11 at 2:23

scenario 1 - you have a good wifi reception - phone dont have to try hard to fetch network, so battery life is longer. but if wifi is tough to connect then offcourse the phone needs to work hard.. so battery drain quicker.

scenario 2 - you using data network, but in bad reception area then it would have same affect on your battery - drained faster..

in the end, depend on your area's connection in order to have longer battery life..
who knows which connection is the best in tht particular area. just simple logic. no technical wonders can save your battery life i say..